Features: - Tanker Operator

not specific and does not tell the analyst what
the hydrocarbon is. In terms of monitoring
tank cleaning, it is very important to know the
nature of the hydrocarbon, because this can
directly impact on the subsequent cleaning
steps. It could be:
i) Previous cargo.
ii) Tank cleaning chemicals.
iii) Old cargo absorbed into coated surfaces.
iv) Reaction of the wall wash solvent with the
coating.
v) Other - for example hydraulic oil, leaking
adjacent cargo etc.
Different hydrocarbon products also have
different responses to the hydrocarbon test,
depending on the relative solubility of the
hydrocarbon in water. The most common
Five of eight 5,600 dwt double
hull chemical tankers delivered to
Bergen-based Mowinckel are
fitted with a Norclean super
stripping system. The other three
are still under construction.
According to Mowinckel Ship Management's
Ove Berntzen, the advantage in fitting the
systems is the reduced time and work needed
for tank cleaning. "In many cases there is no
need to gas free the vessel and go down to
collect the remaining liquid", Berntzen said.
"When we are using the super stripping
system, the remaining liquid is only 0.5-1 litre
in each bilge well. If, for instance, the
remaining cargo is gasoline, we just run the
nitrogen system to clear out the remaining
liquid," he explained.
For these types of vessels, the requirements
for stripping is a maximum of 75 litres
hydrocarbon products to slip under the net of
the hydrocarbon test are aromatics, because
many aromatics have a very slight solubility
in water.
For example toluene has a solubility in
water of approximately 0.18%, meaning that if
the wall wash sample contained for example
0.175% toluene, it would pass the
hydrocarbon test. Clearly having 1,750 ppm of
toluene in a wall wash sample is not the
preferred objective, yet this is quite possible.
Accurately monitoring tank cleaning goes
beyond just 'passing' a wall wash sample and
if done correctly, the cargo tanks will actually
be much cleaner than passing an independent
wall wash inspection.
The responsibility for the tank cleaning is
remaining in each tank with the associated
piping system.
He said that during stripping tests on board
one of the sisterships, the remaining cargo was
found to be 24-50 litres. "As we comply with
the stripping requirements by the use of
'normal' stripping, we do not use the super
stripping system during normal unloading
operations", he explained.
Each ice class tanker has a cargo carrying
capacity at 98% of 6,350 cu m and 138 cu m
of slops and has 13 cargo segregations each
with a double valve. They are fitted with
eight 200 cu m per hour, four 150 cu m
per hour and one 100 cu m per hour
centrifugal pumps.
They are provided with a cargo stock on
each side and a common line of 200 mm port
and starboard for and aft of the manifold.
All the tanks can be discharged through any
TECHNOLOGY - TANK CLEANING
always on the vessel and in the case of a cargo
tank rejection or cargo contamination claim
there is no recourse on the choice of tank
cleaning guide or the validity of the
independent load port inspection. The vessel
has to know how clean the cargo tanks are
before any cargo is loaded and without
monitoring the tank cleaning process this
TO
is impossible.
*This article was written by Guy
Johnson, BSc (Hons) MRSC CChem
CSci), director L&I Maritime (UK)
operations@limaritime.com
Tel. +44 1909 532 003
Super stripping system fitted on board small chemical tankers
of the common lines and/or separately through
the manifold. A stern line of 200 mm is also
fitted. Each presentation flange from the COT
is of DIN standard and 150 mm.
The entire cargo system is constructed of
stainless steel and the vessels are coated with
MarineLine. As well being fitted with the
Norclean super stripping system, a nitrogen
system was also installed in the tanks for
inerting and purging.
Out of the eight DNV classed vessels, the
Frosta, Troma, Lycian, Lydian and Ionian
have been delivered, while the remaining
three - Hadra, Hitra and Vinga will follow
this year and next. They are all products of
the Celik Tekne Shipyard in Tuzla, near
Istanbul.
Of the vessels in service, Frosta is chartered
to Esso Norge, Troma, Lycian and Lydian to
StatoilHydro and the Ionian to Gefo. TO
Left: The stripping requirement is a maximum of 75 litres for this type of vessel. Right: Troma is long term chartered to StatoilHydro.
August/September 2008 � TANKEROperator 59